(a). Field of the Invention
The present invention relates in general to a multi-carrier communication system, and more particularly to a compensation apparatus and method for sampling and carrier frequency synchronization in a multi-carrier communication system.
(b). Description of the Prior Arts
Multi-carrier modulation is one of wideband technologies and increasingly important in recent years. FIG. 1 is a block diagram of a communication system 100 using multi-carrier modulation. The communication system 100 employs a set of N-point inverse fast Fourier transform (IFFT) 102 in the transmitter and the fast Fourier transform (FFT) 111 in the receiver to transceive data. A channel 106 is divided into N sub-channels, and signals of one sub-channel are orthogonal to those of any other sub-channels. Thus, the data transmitted in the sub-channels would not interfere each other, and inter-channel interference (ICI) can be avoided.
The set of N-point outputting from IFFT is called a symbol. To avoid inter-symbol interference (ISI) and ICI, it is common to add a “cyclic prefix” (CP) to each symbol, i.e. the last υ samples of each symbol are copy and added in the front of the symbol. Therefore, a symbol including (N+υ) samples is transceived each time. The circuits 103 in the transmitter and 110 in the receiver of FIG. 1 are used to add and remove cyclic prefixes respectively.
However, the conventional communication system 100 of FIG. 1 suffers the disadvantage of frequency offset. An analog-to-digital converter (ADC) 107 is used to sample signals with a sampling frequency {circumflex over (f)}s in the receiver. However, the sampling frequency fs of the digital-to-analog converter (DAC) 105 in the transmitter is inconsistent with the sampling frequency {circumflex over (f)}s at the receiver. Therefore, ICI is generated in the output of the FFT circuit 111, and a symbol timing error is also increased over time. The performance of the communication system is thus degraded seriously.
The conventional approach to overcome the above-disclosed problem is to utilize the delay-rotor property of the communication system 100. FIG. 2 is a diagram illustrating the delay-rotor property of the conventional communication system. Since the sampling frequency of the transmitter and that of the receiver are different, i.e. fs−{circumflex over (f)}s=dfs≠0, the symbol timing error will be accumulated. This timing error would cause the output signal of the FFT circuit 111 an additional angle rotation in the frequency domain. This rotation angle would also be increased over time, as shown in FIG. 2.
By utilizing the delay-rotor property, the communication system 100 may reserve a specific sub-channel for transmitting a given signal called pilot tone. When the receiver detects an additional angle rotation of the pilot tone in the frequency domain, the symbol timing error resulted from the sampling frequency offset can be estimated. FIG. 3 is a block diagram of a conventional architecture for dealing with the sampling frequency offset. As shown in FIG. 3, after the circuit 301 selects the pilot tone, the angle estimator 302 estimates the angle of frequency-domain signals and passes it to the first difference device 303 (i.e. 1−z−1) to calculate the additional angle rotation. Next, after being provided into the loop filter H(z) 304 and then the accumulator 305
      (          i      .      e      .                          ⁢              1                  1          -                      z                          -              1                                            )    ,the angle rotation is fed back to a voltage-controlled crystal oscillator (VCXO) 306 to compensate the sampling frequency offset of the receiver.
However, the relationship between the sampling frequency offset and the rotation angle is not clear, it is difficult to design the loop filter 304. Besides, it takes a very long time for the feedback loop of FIG. 3 to track the sampling frequency offset. If the sampling frequency is time-variant, then the tracking performance would be impacted. Even worse that the feedback loop cannot track the sampling frequency offset.
In view of this, the present invention provides an apparatus and associated method that can track and compensate the sampling frequency offset more quickly, and tracking delay resulted from the feedback loop delay is also decreased compared to the conventional method. Meanwhile, the system can still maintain the low complexity as the conventional system.